Steve's sheer helplessness is the best part, he *thinks* he understands why Bucky needs to eat like crazy so he's trying to help him do that, and he's really confused when Bucky switches to nearly starving himself but he thinks he understands the compulsion to stay fighting-fit so he tries to help Bucky do that. And the more Steve tries to be helpful and supportive, the more he's just enabling the whole cycle! I bet sometimes Steve is even the one to set it off, like if Bucky's at the stage where he's eating like crazy, Steve makes some kind of comment about Bucky's appetite and it makes Bucky realize what he's been doing and after that he refuses to eat for like three days.